Americans spent $1.5 billion on cold and flu supplements and other "immunity boosters" in 2007.

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Bob Martin says he hasn't caught a cold in years, not since he discovered echinacea and goldenseal. For the past decade, the 60-year-old teacher from Placitas, N.M., has taken the herbal supplements in megadoses three times a day at the first sign of a sniffle. Now he only gets sick if he doesn't dose himself in time, he says.

"It's been years since I've had a cold," says Martin. "I take the herbs, climb under the blanket, and I'm fine the next day. They nip it in the bud."

Martin is not alone in his faith in herbal remedies, which he also takes for toothaches, earaches, and other ailments. According to the Nutrition Business Journal, Americans spent $1.5 billion on cold and flu supplements and other "immunity boosters" in 2007, and the market is growing at twice the rate as that of the standard, over-the-counter, cough-and-cold-remedy market.

But do these alternative treatments actually work?

Martin and plenty of other consumers answer a resounding "yes," but experts say that, overall, there's little evidence-based scientific literature on the subject. In fact, the makers of Airborne, the ubiquitous "effervescent health formula," settled a class-action lawsuit to the tune of $23.3 million in 2008. The charge? The Center for Science in the Public Interest, which helped litigate the suit, and the Federal Trade Commission, which filed a separate complaint against the manufacturer, said the company made false and unsubstantiated claims when it said its product could fight germs or prevent colds.

"Only a very, very small number of compounds have undergone peer review," says Frank Esper, MD, member of the division of pediatric infectious diseases at Rainbow Babies and Children's Hospital, in Cleveland. "What you see is anecdotal."

And the ones that have undergone peer review generally come back with mixed reviews.

The evidence is often not strong, and it can be conflicting. "But there is some good evidence that some of these things can be effective," says David Leopold, MD, director of integrative medical education at the Scripps Center for Integrative Medicine and a faculty member with the Scripps Natural Supplement Conference, in La Jolla, Calif.

Herbal remedies don't seem to prevent colds, but they may help curb symptoms or shorten their duration, he says. "The things I talk about will reduce duration 24 or 36 hours, which is significant if you're out doing things," Dr. Leopold says. "They also seem to decrease severity of symptomology."